1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to a method of horizontally and vertically removing offsets generated between channels at the same time.
2. Description of the Related Art
In general, an LCD device is constructed with a liquid crystal panel unit and a driving unit. The liquid crystal panel unit is constructed with a lower glass substrate in which pixel electrodes and thin film transistors are arranged in a matrix form, an upper glass substrate constructed with common electrodes and a color filter layer, and a liquid crystal layer inserted between the upper and lower glass substrates. The driving unit includes an image signal processing unit for processing an image signal that is externally input and outputting a composite synchronization signal, a control unit for receiving the composite synchronization signal that is output from the image signal processing unit, separately outputting a horizontal synchronization signal and a vertical synchronization signal, and controlling timing according to a mode selection signal, and gate and source drivers for sequentially applying a driving voltage to scan lines and signal lines of the liquid crystal panel unit in response to an output signal of the control unit.
In the LCD device, voltages applied to pixels s required to be inverted. When electric field with single polarity is applied for a long time, deterioration of a liquid crystal material or oriented layer or parasitic charge due to impurities occurs. Accordingly, this operation is performed so as to prevent deterioration in display quality such as image persistence.
In order to prevent deterioration of pixels, polarities of pixels have to be inverted for each frame. Flickers of the liquid crystal panel occur due to a small difference in luminance between polarities. Driving methods such as a row inversion driving method, a column inversion driving method, a dot inversion driving method, and the like are used to reduce the flickers. In the row inversion driving method, the pixels are driven so that neighboring gate lines are inversely displayed with respect to each other in negative and positive polarity combination of the liquid crystal. In the column inversion driving method, the pixels are driven so that neighboring data lines are inversely displayed with respect to each other. In the dot inversion driving method obtained by combining the row inversion driving method with the column inversion driving method, the pixels are driven so that neighboring pixels surrounding a pixel are inversely displayed with respect to the pixel.
These methods are used to reduce differences in luminance between pixels in a predetermined area by using a fact that human eyes concurrently recognize a plurality of pixels. In general, it is known that the dot inversion driving method is the most valid method that is convenient for a user. The dot inversion driving method is most widely used as an inversion driving method of the LCD device.
On the other hand, in the LCD device, since offsets between channels of the source drivers of the LCD device are important in characteristics of the LCD device, a method for reducing the offsets is actively developed. A cause of the offsets between channels of the source drivers exists in output buffers of the source drivers.
FIGS. 1 and 2 illustrate an output buffer used for a conventional method of removing offsets. Referring to FIG. 1, an output buffer 10 includes a first NMOS transistor M1 including a gate connected to a first input signal IN and a second NMOS transistor M2 including a gate connected to a second input signal INB. A first PMOS transistor M3 is connected between a source voltage VDD and the first NMOS transistor M1. A second PMOS M4 is connected between a source voltage VDD and the second NMOS transistors M2. Gates of the first and second PMOS transistors M3 and M4 are connected to a drain of the second PMOS transistor M4 so as to construct a current mirror. A third NMOS transistor M5 including a gate connected to a bias signal BIAS is connected between the first and second NMOS transistors M1 and M2 and a ground voltage VSS. The output buffer 10 further includes third and fourth NMOS transistors M6 and M7 which are serially connected between the source voltage VDD and the ground voltage VSS. A gate of the third PMOS transistor M6 is connected to a drain of the first NMOS transistor M1 and a drain of the first PMOS transistor M3. A gate of the fourth NMOS transistor is connected to the bias signal BIAS. A drain of the third PMOS transistor M6 and a drain of the fourth NMOS transistor M7 output an output signal.
Offsets of the output buffer 10 is caused by a mismatch of the first and second NMOS transistors M1 and M2 which are differential pair transistors and a mismatch of the first and second PMOS transistors M3 and M4 which are active load transistors. Mismatches of the aforementioned transistors M1 to M4 are caused in a procedure of fabricating the transistors included in the process of fabricating a semiconductor device. The offsets are direct current (DC) offsets. The offsets arbitrarily occur.
When the offsets occur, an input level of the output buffer 10 is different from an output level, thereby causing a brightness difference of the liquid crystal panel. The first type output buffer 10 of FIG. 1 and a second type output buffer 20 of FIG. 2 are used to compensate the brightness difference. In FIG. 1, the first type output buffer 10 in which the second input signal INB and the output signal OUT are connected to each other is embodied. The first type output buffer 10 has a positive offset. In FIG. 2, the second type output buffer 20 is illustrated. In the second type output buffer 20, a second input signal INB is connected to a gate of a first NMOS transistor M1. A first input signal IN is connected to a gate of a second NMOS transistor M2. Gates of first and second PMOS transistors M3 and M4 which constitute a current mirror are connected to a drain of the first PMOS transistor M3. A gate of the third PMOS transistor M6 is connected to a drain of the second PMOS transistor M4. The second type output buffer 20 has a negative offset.
If the differential pair transistors M1 and M2 and the active load transistors M3 and M4 are alternately switched by using the first type output buffer 10 and the second type output buffer 20, as shown in FIG. 3, when the input signal IN is about 5 V, the output signal OUT of the first output buffer 10 is about 5.1 V. When the output signal OUT of the second output buffer 20 is about 4.9 V, the output signal OUT of the second output buffer 20 is about 4.9 V. Accordingly, the mean output signal OUT is about 5.0 V that is a mean value in which positive and negative offsets are compensated. Thus, a brightness difference of the liquid crystal panel disappears.
FIG. 4 illustrates a conventional method of removing offsets in a vertical 1-dot inversion driving method. Referring to FIG. 4, output lines of a source driver are denoted by S1 to S6. Gate lines of a gate driver are denoted by G1 to G4. For the convenience of indication, the first type output buffer 10 is denoted by A, and the second type output buffer 20 is denoted by B. In the vertical 1-dot inversion driving method, the first type output buffer 10 and the second type output buffer 20 are alternately arranged in units of two rows. Accordingly, the offsets are vertically removed. However, the offsets are not horizontally compensated. A horizontal two-line dim phenomenon in which two lines are bright and two lines are dark occurs. In order to prevent the horizontal two-line dim phenomenon, the first and second type output buffers 10 and 20 are changed each other in units of a frame. If the offsets are large, the entire screen may be flickered.
FIG. 5 illustrates a conventional method of removing offsets in a vertical 2-dot inversion driving method. Referring to FIG. 5, in the vertical 2-dot inversion driving method, the first and second type output buffers 10 and 20 are alternately arranged in units of a row. Accordingly, the offsets are vertically removed. However, the offsets are not horizontally compensated. A horizontal one-line dim phenomenon in which a line is bright and a line is dark occurs. In order to prevent the horizontal one-line dim phenomenon, the first and second type output buffers 10 and 20 are changed each other in units of a frame. If the offsets are large, the entire screen may be flickered.
FIG. 6 illustrates a conventional method of removing offsets in a horizontal 2-dot inversion driving method. Referring to FIG. 6, in the horizontal 2-dot inversion driving method, the first and second type output buffers 10 and 20 are alternately arranged in units of two rows. Accordingly, the offsets are vertically removed. However, the offsets are not horizontally compensated. A horizontal two-line dim phenomenon in which two lines are bright and two lines are dark occurs. In order to prevent the horizontal two-line dim phenomenon, the first and second type output buffers 10 and 20 are changed each other in units of a frame. If the offsets are large, the entire screen may be flickered.
FIG. 7 illustrates a conventional method of removing offsets in a square inversion driving method. Referring to FIG. 7, in the square inversion driving method obtained by combining the horizontal 2-dot inversion driving method with the vertical 2-dot inversion driving method, the first and second type output buffers 10 and 20 are alternately arranged in units of two rows. Accordingly, the offsets are vertically removed. However, the offsets are not horizontally compensated. A horizontal one-line dim phenomenon in which a line is bright and a line is dark occurs. In order to prevent the one-line dim phenomenon, the first and second type output buffers 10 and 20 are changed with each other in units of a frame. If the offsets are large, the entire screen may be flickered.
In the methods of removing the offsets of FIGS. 4 to 7, the offsets are vertically removed, but the offsets are not horizontally removed.